The exponential decay of the strength of magnetic fields dictates that, in induction welding processes, the structure closest to the induction coil will be the hottest, since it experiences the strongest field. Therefore, it is difficult to obtain adequate heating at the bond line between two graphite or carbon fiber reinforced resin matrix composites relying on the susceptibility of the fibers alone as the source of heating in the assembly. For the inner plies to be hot enough to melt the resin, the outer plies closer to the induction coil and in the stronger magnetic field are too hot. The matrix resin in the entire piece of composite melts. The overheating results in porosity in the product, delamination, and, in some cases, destruction or denaturing of the resin. To avoid overheating of the outer plies and to insure adequate heating of the inner plies, a susceptor of significantly higher conductivity than the fibers is used to peak the heating selectively at the bond line of the plies when heating from one side. An electromagnetic induction coil on one side of the assembly heats a susceptor to melt and cure a thermoplastic resin (also sometimes referred to as an adhesive) to bond the elements of the assembly together. Often the current density in the susceptor is higher at the edges of the susceptor than in the center because of the nonlinearity of the coil. This problem typically occurs when using a cup core induction coil like that described in U.S. Pat. No. 5,313,037 and can result in overheating the edges of the assembly or underheating the center, either condition leading to inferior welds because of non-uniform curing. It is necessary to have an open or mesh pattern in the susceptor to allow the resin to bond between the composite elements of the assembly when the resin heats and melts.
Three major joining technologies exist for aerospace composite structure: mechanical fastening; adhesive bonding; and welding. Both mechanical fastening and adhesive bonding are costly, time consuming assembly steps that introduce excess cost even if the parts that are assembled are fabricated from components produced by an emerging, cost efficient process. Mechanical fastening requires expensive hole locating, drilling, shimming, and fastener installation, while adhesive bonding (i.e., gluing the parts together) requires complicated surface pretreatments, and produces a distinct interface between the part and adhesive rather than a fusion bond which welding produces.
Thermoplastic welding, which eliminates fasteners, features the ability to join thermoplastic composite components at high speeds with minimum touch labor and little, if any, pretreatments. In our experience, the welding interlayer, called a susceptor, also can simultaneously take the place of shims required in mechanical fastening. As such, composite welding holds promise to be an affordable joining process. For "welding" thermoplastic and thermoset composite parts together, the resin that the susceptor melts functions as a hot melt adhesive. If fully realized, the thermoplastic-thermoset bonding will further reduce the cost of composite assembly.
There is a large stake in developing a successful induction welding process suitable for making aerospace composites. Its advantages versus traditional composite joining methods are:
reduced parts count versus fasteners PA1 minimal surface preparation, in most cases a simple solvent PA1 wipe to remove surface contaminants PA1 indefinite shelf life at room temperature PA1 short process cycle time, typically measured in minutes PA1 enhanced joint performance, especially hot/wet and fatigue PA1 permits rapid field repair of composites or other structures.
There is little or no loss of bond strength after prolonged exposure to environmental influences.
Thermoplastic welding is a process for forming a fusion bond between two faying thermoplastic faces of two parts. A fusion bond is created when the thermoplastic on the surface of the two parts is heated to its melting or softening point and the two surfaces are brought into contact, so that the thermoplastic mixes together, and the surfaces are held in contact while the thermoplastic cools below the softening temperature. There is no distinct interface in the finished part.
Simple as the process sounds, and easy as it is to perform in the laboratory on small pieces, it becomes difficult to perform reliably and repeatably in a real factory on full-scale parts to build a large structure such as an airplane wing box. The difficulty is in getting the proper amount of heat to the bond line without overheating the entire structure and also in achieving intimate contact of the faying surfaces of the two parts at the bond line during heating and cooling despite the normal imperfections in the flatness of composite parts, thermal expansion of the thermoplastic during heating to the softening or melting temperature, flow of the thermoplastic out of the bond line under pressure, and then contraction of the thermoplastic in the bond line during cooling.
One technique for getting heat to the bond line in a thermoplastic welding assembly is to include a conductive layer or article, known as a "susceptor," between the two surfaces to be welded, and to heat the susceptor by resistive heating so that the susceptor functions as a bond line heat source to melt or soften the thermoplastic at the bond line for fusion of the faying surfaces of the two surfaces. The electric current for heating the susceptor can be in the form of eddy currents generated inductively, as taught for example by U.S. Pat. Nos. 3,395,261 and 4,978,825, or it can be conducted directly to the susceptor through tabs or the like as shown in U.S. Pat. No. 5,313,034.
U.S. Pat. No. 4,673,450 describes a method to spot weld graphite fiber reinforced PEEK composites using a pair of electrodes. After roughening the surfaces of the prefabricated PEEK composites in the region of the bond, Burke placed a PEEK adhesive ply along the bond line, applied a pressure of about 50-100 psi through the electrodes, and heated the embedded graphite fibers by applying a voltage in the range of 20-40 volts at 30-40 amps for approximately 5-10 seconds with the electrodes. Access to both sides of the assembly is required in this process which limits its application.
Additional techniques for thermoplastic welding with induction heating are illustrated in U.S. Pat. No. 3,996,402 and 4,120,712, where the metallic susceptors of conventional type have a regular pattern of openings of traditional manufacture, being generally circular holes in the metal sheet with a relatively small open area fraction. Achieving a uniform, controllable temperature in the bond line, which is crucial to preparing a thermoplastic weld of adequate integrity to permit use of welding in aerospace primary structure, is difficult with conventional coils or conventional susceptors, as we discussed and illustrated in our copending U.S. patent application Ser. No. 08/068,520.
In U.S. patent application Ser. Nos. 08/286,360 and 08/068,520, we described a tailored susceptor for approaching the desired temperature uniformity. This susceptor relied upon carefully controlling the geometry of openings in the susceptor (both their orientation and their spacing) to distribute the heat evenly. For example, we suggested using a regular array of anisotropic, diamond shaped openings with a ratio of the length (L) to the width (W) greater than 1 to provide a superior weld over that achieved using a susceptor having a similar array, but one where the L/W ratio was one. By changing the length to width ratio (the aspect ratio) of the diamond-shaped openings in the susceptor, we achieved a large difference in the longitudinal and transverse conductivity in the susceptor, and, thereby, tailored the current density within the susceptor. A susceptor having openings with a length (L) to width (W) ratio of 2:1 has a longitudinal conductivity about four times the transverse conductivity so current is more likely to flow longitudinally in the susceptor rather than transversely. In addition to tailoring the shape of the openings to tailor the susceptor, we altered the current density in regions near the edges by increasing the foil density (i.e., the absolute amount of metal). Increasing the foil density along the edge of the susceptor increases the conductivity along the edge and reduces the current density and the edge heating. We increased foil density by folding the susceptor to form edge strips of double thickness or by compressing openings near the edge of an otherwise uniform susceptor. We found these susceptors difficult to reproduce reliably. Also, their use forced careful placement and alignment along the bond line to achieve the desired effect.
Our tailored susceptor was designed for use with the cup core of U.S. Pat. No. 5,313,037. With this coil, the magnetic field is strongest near the edges of the susceptor because the central pole creates a null in the center. Therefore, the tailored susceptor is designed to counter the field by accommodating the higher induced current near the edges. The high longitudinal conductivity encourages induced currents to flow longitudinally. The cup coil of U.S. Pat. No. 5,313,037 creates a strong magnetic field ahead of and behind the coil as it travels along the bond line, which functions to preheat the bond line ahead of the coil or to slow the cooling behind the coil. The slowed cooling allows us to keep the thermoplastic molten while we press the parts together with the trailing pressure plate of a skate that carries the coil in the welding operation. We obtain a strong weld. Preheating with the field ahead of the coil also seems to provide improved results. The welding process and equipment, especially the skate, are more fully described in copending U.S. patent application Ser. No. 08/352,991 by John Mittleider et al., which we incorporate by reference. Alignment of the coil over the tailored susceptor is critical to achieve quality results, but precision alignment within about 0.040 in is difficult to achieve and to maintain.
The selvaged susceptor for thermoplastic welding that we described in U.S. patent application Ser. No. 08/314,027 controls the current density pattern during eddy current heating by an asymmetric induction work coil of the present invention to provide substantially uniform heating to a composite assembly and to insure the strength and integrity of the weld in the completed part. This selvaged susceptor is particularly desirable for welding ribs between prior welded spars because of the control of the heating that we achieve with the asymmetric induction work coil. While designed for the asymmetric work coil, the selvaged susceptor can also be used with the cup coil where it allows wider misalignment between the coil and susceptor because of the high conductivity selvage edge strips.
The power (P) or power density which the susceptor dissipates as heat follows the well-known equation for power loss in a resistor: P=(J.sup.2)(R) wherein J is the eddy current (or its density) and R is the impedance (i.e., resistance) of any segment of the eddy path. The heating achieved directly corresponds to the power (or power density).
We achieve better performance (i.e., more uniform heating) in rib welding by using a selvaged susceptor having edge strips without openings. This susceptor is described in more detail in U.S. patent application Ser. No. 08/314,027. It has a center portion with a regular pattern of openings and solid foil edges, which we refer to as selvage edge strips. We embed the selvaged susceptor in a thermoplastic resin to make a susceptor/resin tape that is easy to handle and to use in assembling (i.e., preforming) the composite pieces prior to welding. Also, we have discovered that, with a selvaged susceptor, the impedance of the central, mesh or grid portion of the susceptor should be anisotropic with a lower transverse impedance than the longitudinal impedance. Here, the L/W ratio of diamond shaped openings should be less than or equal to one. That is, unlike our tailored susceptor, L should be less than W in the selvaged susceptor. With this new selvaged susceptor we encourage the current to flow across the susceptor to the edges where the current density is lowest and the conductivity, highest, and the current flow remains primarily under the coil because of the asymmetric field. We achieve reliable welds, especially on ribs, because the asymmetric induction work coil of this invention generates a magnetic field of high field strength between its poles and low field strength outside its poles. Containing the magnetic field between the poles allows us to direct the heating to a precise location and allows us to complete the rib welds at the beginning and end of runs without remelting the thermoplastic under the spars. Ahead of the coil, there is a magnetic field of low field strength to preheat the susceptor, similar to the cup coil. Behind the asymmetric coil, there is essentially no magnetic field, which is how we achieve controlled heating at the beginning and ending of welding runs.
For the production of complex aerospace structure, such as composite wingboxes in which composite skins are joined to longitudinal spars and transverse ribs, completing rib welds is troublesome with a cup coil like the one in U.S. Pat. No. 5,313,037. The "cup" coil produces a magnetic field that spreads too far ahead of and too far behind the coil so that, in trying to ensure that the edges of the rib at the beginning or end of a run were fully welded, the magnetic field with the "cup" coil induces substantial currents in the susceptors over the spars and resoftens those welds. To overcome this problem we use the asymmetric induction work coil that permits complete welding of the ribs without softening occurring over the adjoining spars.